Transforming growth factor beta 1 (TGF-b1) induces growth suppression in a variety of cell types. The signaling molecules that are responsible for transmitting TGF-b signal include Smad2, 3, and 4. Upon TGF-b engagement, Smad2 and 3 become phosphorylated and form heteromeric complexes with Smad4 and translocate to nucleus, where in association with transcriptional co-activators or repressors, binds to the promoters of TGF-b-responsive genes. In our laboratory, we are studying a diffuse large B-cell lymphoma cell line, DB that lacks TGF-b responsiveness with respect to growth suppression. Our goal is to identify the deficit(s) in TGF-b signaling pathway in DB cells. Preliminary data indicate that the nuclear retention of phospho-Smad3 upon TGF-b treatment is transient in DB cells compared to a TGF-b-responsive B cell lymphoma cell line where the retention of phospho-Smad3 is sustained. Interestingly, the nuclear retention of phospho-Smad2 is similar in both cell types. Ectopic expression of wild type Smad3 is being tested to reverse the deficit in TGF-b responsiveness. We are currently investigating the mechanism behind the transient retention of nuclear phospho-Smad3, and how this transient retention correlates to the lack of TGF-b-induced growth suppression.